1. Field of the Invention
This invention relates to a shift indicator system for a vehicle, and more particularly to a shift indicator system for a vehicle for indicating to the driver the timing to shift so that an optimal fuel economy can be obtained while maintaining a desired running performance.
2. Description of the Prior Art
In the case of a vehicle provided with a manual transmission, the driver shifts at timings determined at his own judgment so that a desired running condition is obtained. However, if the shifting timing is not appropriate, e.g., if a lower gear speed is kept in an operating range of the engine in which a sufficient driving force can be obtained even in the next higher gear speed, fuel economy is reduced. Thus, there have been proposed shift indicators for directing the driver to shift when it is preferable to do so in view of the fuel economy. See Japanese Unexamined Patent Publication Nos. 59(1984)-26332, and 59(1984)-29519. In the shift indicators, there is defined an engine operational range in which the engine speed is higher than a predetermined value and the throttle opening or the engine load is between predetermined upper and lower limits and in which the transmission gear should be shifted up in order to improve fuel economy (such an operational range will be referred to as "upshift range", hereinbelow), and when the actual engine operating condition enters the upshift range, a lamp and/or a buzzer are energized to direct the driver to upshift. The upshift range is determined so that an engine output power sufficient to maintain a desired running performance can be obtained after the upshift. By shifting up the transmission gear when the engine operating condition is in the upshift range, fuel economy can be improved by virtue of reduction of the sliding resistance in various parts of the engine due to reduction of the engine speed, reduction of the pumping loss due to increased throttle opening, and the like.
However, if the upshift range is fixedly set in the shift indicator system, the upshifting indication does not conform to the driver's feeling depending on the engine operating condition, or reduction of fuel consumption cannot be obtained when the driver upshifts following the indication of upshifting depending on the engine operating condition. For example, if the upshifting indication is made when the driver intends to accelerate the vehicle by pushing down the accelerator pedal with the transmission kept in a low speed, the indication deviates from the intention of the driver, thereby adversely affecting the reliability and properness of the indication. Further, for example, if the transmission is shifted up following the indication of upshifting when the engine speed is abruptly increased over the preset speed in response to acceleration at a low gear speed and the upshifting indication is provided, the engine speed will exceed the preset value by a large amount before the transmission is actually shifted up and accordingly fuel consumption reduction cannot be fully enjoyed.
In Japanese Unexamined Patent Publication No. 59(1984)-29519, it is proposed to change the upshift range depending on the operating condition, especially to change the preset engine speed above which the upshifting indication is to be provided during acceleration of the vehicle. That is, as the acceleration of the engine speed is increased, the preset engine speed is raised to delay providing the upshifting indication. Though this is to avoid a situation in which indication for directing upshift, for instance, from second to third is provided immediately after upshifting from first to second so that the driver cannot follow the indication in the case that the acceleration of the engine speed is very high, this also contributes to prevention of a premature upshifting indication conflicting with the driver's feeling when the driver abruptly pushes down the accelerator pedal to accelerate the vehicle.
Further, in Japanese Unexamined Patent Publication No. 59(1984)-26332, it is proposed to provide the indication of upshifting earlier when the vehicle is being accelerated by lowering the preset engine speed with increase in the acceleration of the engine speed. This is to prevent the engine speed from exceeding the preset engine speed by a large amount before the transmission is actually shifted up and the engine speed is lowered due to the upshift after the upshifting indication is provided in the case that the acceleration of the engine speed is increased due to a low gear speed. With this arrangement, the engine speed can be prevented from being excessively increased to adversely affect the fuel economy.
However, it is impossible to simultaneously implement both the proposals described in the two Japanese patent publications since when the acceleration of the engine speed is large, the preset speed is raised according to one proposal and is lowered according to the other proposal. Therefore, when the fuel economy is to be improved, prevention of untimely provision of the indication of upshifting must be sacrificed, and vice versa.